Labor leader Bill Shorten has broken his silence about damaging rape allegations dating back to the 1980s after Victorian Police told him there was no case and no charges to be laid.

In a move designed to put the issue behind him, Mr Shorten revealed he was the "senior Labor figure" subject to an investigation that had been concluded, asserting "there is absolutely no basis to the claims".

"Others who are aware of the investigation have acted with the utmost integrity by leaving the police to do their job."

The investigation began in November last year after a complaint was made to police by a woman who now lives on the NSW central coast.

The woman's allegation were made on Facebook and at the time, it was reported that she had written on the social networking site: "In 1985 I joined the ALP. In 86 at the age of 16 I ... became a delegate for state and national conferences. In 86 I went to a Young Labor camp down near Geelong ... I was alone ... at about 4am there was a knock at my door. It was him at the door. He pushed me into a bathroom, up against a towel rail, pulled down my pants and raped me."

Victoria Police issued a statement about the results of their investigation.

"Victoria Police can confirm that a report of an alleged historical sexual assault has been investigated by police. Investigating police sought advice from the Office of Public Prosecutions, which advised there was no reasonable prospect of conviction. All parties have been notified that Victoria Police will not be proceeding with criminal charges."

Explaining his decision to go public, Mr Shorten said he would not go into the details of the allegation, "except to say that the allegation was untrue and abhorrent. The allegation was made by someone that I knew briefly at that time."

"The police have now concluded the investigation. The decision speaks for itself. It is over. I have no intention of making any further comment," he said.

"In all fairness, I am entitled to draw a line on this."

Mr Shorten said he learned about the allegation, which dates back to when he was 19, late last year. The claim had been made on social media, he said, soon after he had been elected Opposition Leader.

On Friday, Prime Minister Tony Abbott told ABC radio Adelaide the matter was from "many, many years ago and it's a personal matter and I think he's dealt with it".

"Obviously when something like that is swirling around in the ether sometimes it's best to deal with them openly and he's done what he thinks is best," he said.

Liberal cabinet minister Christopher Pyne said the decision to go public was a matter for Mr Shorten and that he "obviously made that judgment call".

"I'm always glad for anyone who has been accused of something, if the accusations turn out to be not true, to be inaccurate and for the police to drop them."

Labor shadow minister Richard Marles, an ally and friend of Mr Shorten's, immediately came forward to defend his party leader.

"It's a difficult step that Bill has taken today, I think it's a courageous step but I know he really wanted to clear the air on this issue," he said.

"Anyone who had been on social media would have known that Bill's name's been connected to these allegations. And I think he wanted to set the record straight once and for all, and particularly given that there has now been a thorough and detailed investigation by the Victorian Police which has made it unequivocally clear that there is no case to answer."

Mr Marles, who has known Mr Shorten since 1987, said "I know that the allegations are not true".

"I know they are not true because the person that I've known - a close friend of mine from that time right until now - is not capable of undertaking the allegations that have been made against him."

The Geelong MP said Mr Shorten had first spoken to him about the allegations in November last year, the night before reports were published that an unnamed senior Labor figure was facing sexual assault allegations.

"It came out of the blue and shocked him," Mr Marles said.

On Friday, Liberal frontbencher Malcolm Turnbull supported Mr Shorten's decision to publicly front up to "very, very painful" rape allegations from almost 30 years ago.

Mr Turnbull said Mr Shorten did the right thing in raising the damaging allegations

"I think Bill Shorten made the right decision," the Communications Minister told the Nine Network on Friday.

"It would be very, very painful to feel you are the subject of an unjust accusation."

Deputy opposition leader Tanya Plibersek said her leader decided to draw a line under the events.

"It is good that he can," Ms Plibersek said."It has been really stressful."

Bill Shorten's statement

Late last year I learned that a claim had been made about me, going back to when I was 19. It was made on social media when I was elected Opposition Leader.

I will not go into the details, except to say that the allegation was untrue and abhorrent. The allegation was made by someone that I knew briefly at that time. There is absolutely no basis to the claim.

The claim has now been thoroughly and vigorously investigated by the police as is entirely proper. I fully cooperated to clear my name and that is what I've done.

I freely answered all the questions that the police asked of me. Now the police investigation is concluded, I can make this statement.

The easy option would be to say nothing, but that is not who I am. I want to address this myself directly.

This has been deeply distressing for my family. I'm thankful for the love and support of Chloe and the support of my staff and parliamentary colleagues.

Others who are aware of the investigation have acted with the utmost integrity by leaving the police to do their job.

The police have now concluded the investigation. The decision speaks for itself. It is over. I have no intention of making any further comment. I'm happy to take questions.

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